1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image of a document (referred to as a document image hereinafter) and converting the read image into an image signal, and more particularly, to an image reading apparatus controllable by an external apparatus such as a data processing apparatus, a controller of an image filing system, a host computer, a microcomputer, or the like.
2. Description of the Prior Art
Conventionally, there has been suggested an image reading apparatus for reading image information such as characters, a document image or the like which is typed or printed on a piece of paper, converting the read image information into digital image signal and outputting it to a printer so as to make a hard copy thereof, or outputting the converted digital image signal to an external data processing unit such as a controller of an image filing system, a host computer, a microcomputer or the like and displaying an image thereof on a display unit connected thereto. As the image reading apparatus of this type, there has been suggested, for example, in U.S. Pat. No. 4,691,237, an image reading apparatus (referred to as a first conventional apparatus hereinafter) capable of selecting either one of receiving a horizontal synchronizing signal or a vertical synchronizing signal of a digital image signal from an external data processing unit and generating them within an internal circuit thereof upon operating in cooperation with an image output unit such as a printer or the like, or upon operating as an input unit of an image filing system.
In the above-mentioned first conventional image reading apparatus, a horizontal synchronizing signal having a predetermined frequency is generated in the internal circuit thereof, and the horizontal synchronizing signal is used as a synchronizing signal upon transferring a digital image signal to a host computer of a system, and is also used as a horizontal synchronizing signal upon reading image information by scanning it in a main scan direction using an CCD image sensor.
However, in the first conventional image reading apparatus, as described above, since the horizontal synchronizing signal having the predetermined frequency is generated in the internal circuit thereof and the horizontal synchronizing signal is used as not only the synchronizing signal upon transferring the digital image signal to the host computer of the system but also the horizontal synchronizing signal upon reading image information by scanning it in the main scan direction using the CCD image sensor, the image reading apparatus can not respond every horizontal scanning line of the image signal to a request for outputting the image signal or another request for interrupting the operation of reading the image information.
Further, in the first conventional image reading apparatus, since the horizontal synchronizing signal having the predetermined frequency is generated, the output rate of the image signal from a scanner is previously set to a predetermined value. Therefore, in the first conventional image reading apparatus, available external data processing units are limited depending on the output rate of the image signal from the scanner.
Further, in the first conventional image reading apparatus, when performing a process for converting the resolution of the image (referred as a resolution conversion process hereinafter) such as a process for converting an image signal into the image signal having a higher resolution by increasing the image signal of one horizontal scanning line read by the scanner into the image signal of a plurality of horizontal scanning lines, or another process for converting an image signal into the image signal having a lower resolution by decreasing the image signal of a plurality of horizontal scanning lines into the image signal of one horizontal scanning line, the frequency of the horizontal synchronizing signal is changed depending on the contents of the resolution conversion process, and then, as a result, the converted image signal can not be outputted to the external data processing unit. Thus, the first conventional image reading apparatus can not perform the resolution conversion process for the image data read by the scanner.
Further, there is disclosed in U.S. Pat. No. 4,992,888 and has been known to those skilled in the art, an image reading apparatus (referred to as a second conventional apparatus hereinafter), which is not an apparatus for performing the above-mentioned resolution conversion process for the image data read by the scanner, but is capable of performing a magnification process or a density conversion process in a subscan direction perpendicularly to the main scan direction in which sensor elements of the CCD linear image sensor are disposed, by making an reference signal supplied to the linear image sensor synchronous with a driving pulse signal for driving a stepping motor of pulse motor for moving an optical system used for reading a document image and by controlling the driving pulse signal to be applied to the stepping motor.
However, the second conventional image reading apparatus has not only such a problem similar to that of the first conventional apparatus that it can not perform the resolution conversion process but also the following problems. In order to achieve a higher magnification ratio, it is necessary to control the stepping motor in a higher speed, and then it is necessary to design the stepping motor taking into consideration a torque in the maximum speed and margins thereof. As a result, it is necessary to adopt an expensive stepping motor having a relatively high performance, resulting in heightening the manufacturing cost of the image reading apparatus.
In other general conventional image reading apparatus, an image signal having image information for a predetermined time interval in the subscan direction is sent to a data processing unit such as a printer, a computer or the like in synchronous with only a vertical synchronizing signal generated by an internal circuit thereof. Then, the external data processing unit receives and takes therein only the image signal for the predetermined time interval in the subscan direction which is necessary to be processed therein, and then, performs a predetermined image process for the taken-in image signal. Therefore, the conventional image reading apparatus of this type is constituted such that a size of an image and a resolution which can be accepted by the predetermined external data processing unit are previously set, and a vertical synchronizing signal is generated in the internal circuit thereof.
Recently, there have been developed various kinds of external data processing units respectively having a various kinds of performances which are used for processing image data, and the external data processing units may be connected to image reading apparatuses. However, since the conventional image reading apparatus has the above-mentioned composition, it can be connected to only a particular data processing unit having a corresponding predetermined performance. Therefore, for example, even though the specifications of the external data processing unit has been already known, the specifications of the interface of the image signal of the external data processing unit may not coincide with those of the image reading apparatus, and then, the image reading apparatus having been bought already may not be connected to the external data processing unit. In other cases, the specifications of the external data processing unit are not known, and then, it can not be judged whether or not the external data processing unit can be connected to the image reading apparatus.
In these cases, in such a case that a time interval in the subscan direction of the image signal to be processed by the external data processing unit is larger than that of the image signal inputted from the image reading apparatus thereto, or in such another case that the resolution of the image signal to be processed by the external data processing unit is relatively high even though that the time interval in the subscan direction of the image signal to be processed by the external data processing unit is the same as that of the image signal inputted from the image reading apparatus thereto, there may be caused the following trouble. Namely, since the external data processing unit can not receive the image signal, the data processing unit may stop the receiving process still in such a state of taking the image signal therein.